It would be interesting to try the 9-18 behind the 3,5 dome. I would be happy it it was possible to optimize fore the wide end and just use that, skipping zoom. Providing that part of the soft corners are caused by lens nodal point being off the dome's optical center.

As i mentioned: i've been suggested 2 20mm extensions + the 4.33" dome should work with the 9-18. That would be interesting to get a second opinion on.

Cheers

EDIT:

A measurement of the nodal point shift of the 9-18 (and other lenses) can be found here:

I'd be more inclined to try the 9-18mm behind the 4.33" dome with one 20mm (or maybe a 30mm if one was available, i'd think 2x 20mm would be too much). But you'll almost certainly need (or want) a diopter. And it may be the case, like with the 12mm that a strong diopter (e.g. +5) is better than just a small (e.g. +2) correction. In part this is because diopters are less effective at shorter focal lengths. But guessing is little substitute for testing :-)

I should add to this, that the 4" semi-dome that's designed for the 9-18mm is going to be the best port for the lens. However, it may still be significantly better with a +x diopter on the lens.

Great pics! The corners on the 12mm are very soft at f/2.0 behind the 4.33" dome (even at distance). You can use f/8 and get better results, but just buy a $30 B+W 46mm NL5, +5 close-up lens and stick it on the front of the 12mm when you are underwater and the corners will be very sharp even at f/2.0. You won't be able to shoot above water with this setup except at very close distances.

Pre-focus on something about 50-60cm (1.5-2 ft) away from the front of the dome in the air (like your knee or a fin)

Use f/11

Take the shot

If you are shooting something underwater that is very close, then you can drop the in-air pre-focus down to 35-40cm (1.1-1.3 ft) from the front of the dome and use f/13.

In-air pre-focus distances greater than 60cm (2 ft) from the front of the dome have no use.

If you instead focus on something in the water, nothing above water will be in focus unless it's almost touching the dome. And if you focus on something at any distance from the camera above water, nothing underwater will be in focus.

* you need to have your camera configured to separate focus from the shutter ( http://wetpixel.com/...showtopic=48625 ), we still use autofocus to focus, but with the camera in manual focus mode and using the back lever on the Nauticam housing to trigger focus.

Need some expert advise on the use of Nauticam 6" dome port designed for the Panasonic 7-14 mm with an Olympus 9-18 mm. I have read there is no vignetting indicating that there should be no issues, does anyone know of corner sharpness issue of using this larger dome with the 9-18mm lens?